Recycling process water in complex sulphide ore flotation

Sammanfattning: An approach to environmental sustainability and improved process economy in sulphide minerals production is recycling of process water in flotation of complex sulphide ores. However process water chemistry may be a critical issue to flotation efficiency. Process water discharged from sulphide flotation circuits are usually gypsum (CaSO4.H2O) saturated and have a high salinity. The major ionic species contained in the process water are Ca2+ and SO42- ions. As minor species, they commonly contain reduced sulphur compounds (RSC) (sulfoxyanions with sulphur in the oxidation state below (VI) such as SO32-, S2O32-, S2O52-, and S4O62-), cations of ferrous and non-ferrous metals, frothing molecules, residual chemical reagents and products of their degradation. However, the key step towards developing scientific approaches of recycling the process water is elucidation of how, in what extent, and why the recycled water components (taken singly or jointly) influence flotation of sulphides.Therefore a research program was initiated to understand the influence of the major and minor process water species on flotation and how to flexibly control such effects if any. In this work, the influence of major components of calcium and sulphate ions in process water on flotation of chalcopyrite, galena, sphalerite and pyrite minerals were investigated by Hallimond tube flotation. The surface chemistry and xanthate collector adsorption on the minerals under the influence of process water species were also studied by zeta-potential measurement, Fourier transform infrared spectroscopy (FTIR) with diffuse reflectance attachment and Xray photoelectron (XPS) spectroscopy. The effects of the major components were also investigated on complex sulphide ores flotation using bench scale flotation equipment. The impact of the species in flotation was studied using deionised water, tap water, process water and simulated water containing equivalent amount of calcium and sulphate species in process water.Oxidation of sulphide minerals has been known to be a very critical issue during flotation of sulphide minerals while several authors had reported that pyrite, the most abundant metal sulphide and very common gangue mineral in sulphide ores can generate hydrogen peroxide (H2O2). Hydrogen peroxide is a strong oxidizing agent stronger than oxygen. It was revealed that ferric defects on ground pyrite surfaces can generate OH• radicals upon interaction with water and combination of two OH• radicals yields H2O2. However, participation of these species in non-selective oxidation of the sulphide ore pulp components and hence in deteriorating the concentrate grade and recovery of metal-sulphides has not still been explored yet. In an attempt to fill the gap, a preliminary study was done to estimate the concentration of H2O2 in a complex sulphide ore pulp liquid during different times of ore grinding and compare them to oxidation of pulp solid. Study of possible ways of flexibly controlling H2O2 formation through known chemical means was also attempted.Hallimond tube flotation results showed marginally lower recoveries of chalcopyrite and galena in process water and in the presence of calcium and sulphate ions in both deionised and process waters using potassium amyl xanthate as collector. Whereas sphalerite and pyrite floatability were little increased in process water. There is no significant effect of calcium and sulphate ions on sphalerite in deionised water using isobutyl xanthate as collector. Zeta-potential shows the adsorption of calcium ions on the minerals whereby the potential are seen to increase, while sulphate ions have no significant effect. FTIR and XPS studies revealed the presence of surface oxidised sulfoxy species and surface calcium carbonates and/or calcium sulphate on chalcopyrite in the presence of process water and water containing calcium ions. Surface oxidised sulfoxy and carbonate species were seen on galena in the presence of deionised water, process water and water containing calcium and sulphate ions. The studies also showed hydrated surface oxidised species and surface iron and calcium carbonates on pyrite in the presence of process water and water containing calcium ions all at flotation pH 10.5. These surface species influenced xanthate adsorption on chalcopyrite, galena and pyrite. The presence of surface oxidised sulfoxy and carbonate species at the sphalerite flotation pH 11.5 were seen in the presence of deionised water, process water and water containing calcium and sulphate ions, but the surface species does not influence xanthate adsorption.Formation of hydrogen peroxide was revealed during grinding of a complex sulphide ore for the first time in mineral processing application and its formation was reduced by diethylenetriamine (DETA). FTIR spectroscopy of the pulp solid fraction shows varying degree of oxidised surface species which is related to concentration of H2O2 analyzed in pulp liquid. Addition of DETA at low concentration during grinding of complex sulphide ore depresses iron and copper minerals flotation during copper-lead flotation stage, DETA also depresses iron minerals during zinc mineral flotation. Consequently copper minerals flotation to copper-lead products is reduced and they are reporting to zinc products. However zinc minerals flotation to zinc product is improved.Tests performed on two different complex sulphide ores using bench scale flotation equipment showed that chalcopyrite, galena and sphalerite minerals recoveries are better in process water than tap water. The results also showed general decrease of the minerals floatability at temperatures lower than 22oC in either tap water or process water. An analysis of pulp liquid after flotation shows decreased calcium and increased sulphate ions. This indicates that there is adsorption of calcium ions on the minerals and dissolution and release of sulphate ions in solution. The studies showed that the process water can be recycled in flotation with no significant effect on grade and recovery of sulphide minerals.